D-psicose, also called D-allulose, is a rare sugar isomer of fructose. It can be found in nature but at very low concentrations like in edible mushrooms, jackfruit, wheat and the Itea plants.
Unlike fructose, the metabolism of psicose in humans is partly absorbed and metabolized in energy, and partly excreted unchanged in the urine and feces.
The characteristics of D-psicose as a material for preventing lifestyle-related diseases have been disclosed, including its noncaloric nature, a positive effect on the reduction of the glycemic response, an antiobesity effect, and the like. In addition, the sweetness of D-psicose is about 70% of that of sucrose (Oshima, et al. (2006), Psicose contents in various food products and its origin, Food Sci Technol Res 12:137-143), but 0.3% energy of sucrose and is suggested as an ideal sucrose substitute for food products. It can also be used as an inhibitor of hepatic lipogenic enzyme and intestinal α-glycosidase for reducing body fat accumulation. It further shows important physiological functions, such as reactive oxygen species scavenging activity and a neuroprotective effect. In addition, it also improves the gelling behavior and produces good flavor during food processing.
D-psicose exists in extremely small quantities in commercial carbohydrate or agricultural products and is difficult to chemically synthesize. Therefore, interconversion between D-fructose and D-psicose by epimerization using D-tagatose 3-epimerase (DTEase) family enzymes has been confused on as attractive way of D-psicose production.
So far, there have been 9 kinds of DTEase family enzyme sources reported. Twenty years ago, DTEase was first characterized by Izumori et al. from Pseudomonas cichorii, showing C-3 epimerization activity of ketohexoses with the optimum substrate of D-tagatose (Izumori et al. 1993, Biosci. Biotechnol. Biochem. 57, 1037-1039). Till 2006, the second enzyme with C-3 epimerization activity of ketohexoses was identified from Agrobacterium tumefaciens, and it was named D-psicose 3-epimerase (DPEase), due to its high substrate specificity for D-psicose (Kim et al. 2006, Applied and environmental microbiology 72, 981-985; US 2010/0190225; WO2011/040708). Recently, another six DTEase family enzymes were characterized from Rhodobacter sphaeroides SK011 (DTEase) (Zhang et al. 2009, Biotechnology letters 31, 857-862), Clostridium cellulolyticum H10 (DPEase) (Mu et al. 2011, Journal of agricultural and food chemistry 59, 7785-7792, CN102373230), Ruminococcus sp. 5_1_39BFAA (DPEase) (Zhu et al. 2012, Biotechnology letters 34, 1901-1906), Clostridium bolteae ATCC BAA-613 (Jia et al. 2013, Applied Microbiology and Biotechnology, DOI 10.1007/s00253-013-4924-8), Clostridium scindens ATCC 35704 (Zhang et al. 2013, PLoS ONE 8, e62987), and Clostridium sp. BNL1100 (Mu et al. 2013, Biotechnology Letters, DOI 10.1007/s10529-013-1230-6), respectively. In addition, Maruta et al. disclosed a DTEase-producing source in Rhizobium (US 2011/0275138).
There is only one reference to report the enzyme modification of DTEase family enzymes by protein engineering technology. Using random and site-directed mutagenesis technology, Choi et al. (2011, Applied and environmental microbiology 77, 7316-7320) constructed the I33L S213C double-site variant of A. tumefaciens DPEase, and the variant enzyme showed increases in optimal temperature, half-life, melting temperature, and catalysis efficiency, compared with the wild-type enzyme. Its optimal pH remains unchanged at 8.00.
However, the enzymes have optimum pH for activity at 8.0-9.5, and the pH stability is between 8.0-10.0, which is not appropriate for industrial application.
Therefore, the main concern for using psicose remains its scarcity and its production cost, and the need for improved industrial D-psicose production still exists.